Actions of nipecotic acid and SKF89976A on GABA transporter in cone-driven horizontal cells dissociated from the catfish retina.

Whole-cell voltage-clamp recordings were made from dissociated horizontal cells of the catfish retina. In the presence of picrotoxin (PTX, 100 microM), GABA evoked a sustained inward current at negative holding potentials. Dose-response measurements were well fitted by a logistic curve with a Hill coefficient of 1.11 and EC50 of 9.76 microM. When external Na+ was replaced with Li+, this GABA-induced current was eliminated. The substitution of methanesulfonate for Cl- also suppressed the current. This current was blocked by either nipecotic acid or SKF89976A. However, the mechanisms by which these drugs suppress the GABA-induced current differ. Intracellularly applied SKF89976A blocked the GABA-induced current, while nipecotic acid intracellularly had no effect. beta-Alanine at concentrations greater than 1 mM exerted a slight inhibitory effect. Extracellularly applied SKF89976A produced no current by itself but suppressed GABA-induced currents. Dose-response curves showed that SKF89976A has an IC50 of 0.93 microM and a Hill coefficient of 2.68. Nipecotic acid evoked a current response, like GABA. A Hill coefficient was 1.64 and an EC50 was 7.69 microM. This nipecotic acid-induced current was blocked by substituting Li+ for Na+ or by the addition of SKF89976A. This result is consistent with other studies indicating that nipecotic acid is transported in place of GABA. Extracellular Na+ was required for the prolonged suppression by extracellularly applied SKF89976A, while the extracellular Cl- depletion has no influence on the suppression. The pharmacological profile of this GABA transporter fits the neuronal rather than the glial type of cloned transporters.